Explosive compositions containing technical grade 2,4,6-trinitrotoluene (TNT) are often cast into artillery shells, rockets, bombs, etc. by heating the composition to melt the TNT and then pouring the melt into the shell where it is allowed to cool and solidify. Cast charges thus obtained usually exhibit undesirable properties, such as exudation of oily impurities on storage, voids, poor compressive strength, brittleness, lack of homogeneity due to oriented crystal structure of the TNT, etc., which adversely affect ballistic performance and impact sensitivity. Various methods have been proposed to overcome such undesirable properties, but they have been only partially successful. For example, British Pat. No. 1,249,038 discloses that a fine crystalline cast charge of TNT having random crystal structure can be obtained by incorporating a small amount of hexanitrostilbene into the moltem composition. However, the hexanitrostilbene is not effective for overcoming other deficiencies of cast TNT compositions, such as brittleness, exudation, etc. Generally, it has been necessary to employ a combination of different additives to treat the various difficiencies of cast TNT compositions, and the methods used and the results obtained often have not been entirely satisfactory. Additives used in this manner include nitrocellulose and other cellulose esters, e.g. cellulose propionate, natural and synthetic resins, such as waxes, polyurethane resins, epoxy resins, and polyethylene and polypropylene resins.
Recently it has been discovered that the manifold problems and deficiencies associated with cast TNT explosive compositions can be substantially reduced or eliminated by the incorporation of a single, broad spectrum additive consisting of a thermoplastic polyurethane elastomer, which is dissolved in the TNT melt. The method is more fully described in U.S. patent application Ser. No. 66,598, filed 14 Aug. 1979 entitled "Castable TNT Compositions Containing a Broad Spectrum Preformed Thermoplastic Polyurethane Elastomer Additive", H. William Voight, Jr., inventor, which is a continuation application of prior application Ser. No. 885,716, filed 13 Mar. 1978, now abandoned. As noted therein, the thermoplastic polyurethane elastomer additive provides, inter alia, fine random crystallinity of the TNT, reduces exudation, renders the cast TNT stronger, less brittle and more resistant to cracks at low temperatures, permits the cast composition to be remelted and resolidified for reuse without loss of homogeneity and explosive properties and requires no special conditions to exclude the presence of moisture during the melting and casting of the explosive compositions.
The aforementioned application discloses the incorporation of a preferred thermoplastic polymethane elastomer, Estane 5702 (manufactured by the B.F. Goodrich Co. and believed to be the reaction product of 4,4'-diphenylmethane diisocyanate, poly (tetramethyleneadipate) glycol and 1,4-butanediol) in a TNT-based castable explosive composition to produce a modified Composition B explosive consisting of 59.5 parts RDX, 0.5 part Estane 5702 and 40 parts TNT. Estane 5702 and other thermoplastic polyurethane elastomer additives generally dissolve slowly with difficulty in molten TNT in which they have limited solubility. The present method for dissolving the elastomer additive in a TNT melt comprises precoating the elastomer on the crystalline RDX particles by precipitating the elastomer from a solution thereof in an organic solvent, which spreads the elastomer over a large surface and thereby increases the rate of solution of the elastomer in the molten TNT. However, even when thus coated or mixed with the RDX particles, the elastomer dissolves rather slowly in molten TNT, requiring several hours to effect complete solution. Also the method of coating the elastomer on the RDX particles is not entirely satisfactory, since it entails a number of operations, which increase the cost of manufacture and limit the capacity for producing the cast shell filler, which is particularly serious in times of emergency.
To alleviate this problem, an attempt has been made to prepare a concentrate consisting of TNT and the elastomer, e.g. about 2 parts TNT and 1 part Estane 5702, which could be incorporated directly in molten TNT and perhaps dissolve more rapidly therein due to the substitution of soluble TNT in place of RDX, which is essentially insoluble in molten TNT. The method involved dissolving the mixture of TNT and Estane in acetone and evaporating the solvent solution to dryness. Such attempts were unsuccessful, since the product was obtained in the form of a hard, gritty product which dissolved slowly in molten TNT.